Novel group VII histidine kinase HwHhk7B from the halophilic fungi Hortaea werneckii has a putative role in osmosensing.
ABSTRACT Histidine kinases (HKs) are abundant among prokaryotes and have been characterized in fungi and plants, although not yet in animals. These enzymes regulate diverse processes, including adaptation to osmotic stress and virulence of plant and animal pathogens. Here, we report the cloning, characterization and phylogenetic analysis of HwHHK7A and HwHHK7B, HK genes from the fungi Hortaea werneckii, a proposed model system for studying salt tolerance in eukaryotes. The two HwHhk7 isoforms are 96.7% identical in amino-acid sequence and have a typical eukaryotic hybrid HK domain composition. On the bases of the conserved sequence of the H box, they are classified into the group VII ascomycete HKs. For the HwHhk7B protein, the autokinase activity was demonstrated in vitro. The salt-responsive expression of the HwHHK7 genes and the increased osmotolerance of a wild-type Saccharomyces cerevisiae strain expressing the HwHHK7B gene lead us to speculate that these newly identified HKs have roles in osmosensing.
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ABSTRACT: Molecular studies of salt tolerance of eukaryotic microorganisms have until recently been limited to the baker's yeast Saccharomyces cerevisiae and a few other moderately halotolerant yeast. Discovery of the extremely halotolerant and adaptable fungus Hortaea werneckii and the obligate halophile Wallemia ichthyophaga introduced two new model organisms into studies on the mechanisms of salt tolerance in eukaryotes. H. werneckii is unique in its adaptability to fluctuations in salt concentrations, as it can grow without NaCl as well as in the presence of up to 5 M NaCl. On the other hand, W. ichthyophaga requires at least 1.5 M NaCl for growth, but also grows in up to 5 M NaCl. Our studies have revealed the novel and intricate molecular mechanisms used by these fungi to combat high salt concentrations, which differ in many aspects between the extremely halotolerant H. werneckii and the halophilic W. ichthyophaga. Specifically, the high osmolarity glycerol signaling pathway that is important for sensing and responding to increased salt concentrations is here compared between H. werneckii and W. ichthyophaga. In both of these fungi, the key signaling components are conserved, but there are structural and regulation differences between these pathways in H. werneckii and W. ichthyophaga. We also address differences that have been revealed from analysis of their newly sequenced genomes. The most striking characteristics associated with H. werneckii are the large genetic redundancy, the expansion of genes encoding metal cation transporters, and a relatively recent whole genome duplication. In contrast, the genome of W. ichthyophaga is very compact, as only 4884 protein-coding genes are predicted, which cover almost three quarters of the sequence. Importantly, there has been a significant increase in their hydrophobins, cell-wall proteins that have multiple cellular functions.
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ABSTRACT: Although suggested, the involvement of the HOG pathway in adaptation processes in extremely halotolerant fungus H. werneckii has never been specifically demonstrated. Here, we show that the H. werneckii HOG pathway is very robust, and that it includes two functionally redundant MAPK homologues, HwHog1A and HwHog1B, that show osmolyte-type-dependent phosphorylation. Inhibition of HwHog1 kinase activity with the ATP analogue BPTIP restricts H. werneckii colony growth at 3.0 M NaCl, KCl and sorbitol, most likely due to restricted cell division. On the other hand, HwHog1-regulated transcription of a selected group of genes (HwSTL1, HwGUT2, HwOPI3, HwGDH1, HwUGP1, HwGPD1) is an osmolyte-specific process that is important for induction of gene transcription with high NaCl, for regulation of specific genes with high sorbitol, and has no role in KCl stressed cells. Survival of H. werneckii at moderate NaCl and KCl concentrations is not dependent on HwHog1 activity or the calcineurin pathway, and thus alternative mechanisms must exist. The HOG pathway described here is vital for the extreme osmotolerance of H. werneckii, and its regulation shows important differences from the homologue pathways characterised in other mesophilic and halotolerant fungi. Copyright © 2014. Published by Elsevier Inc.Fungal Genetics and Biology 12/2014; 74. DOI:10.1016/j.fgb.2014.11.004 · 3.26 Impact Factor
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ABSTRACT: The production of host-selective toxins by the necrotrophic fungus Alternaria alternata is essential for the pathogenesis. A. alternata infection in citrus leaves induces rapid lipid peroxidation, accumulation of hydrogen peroxide (H2O2), and cell death. The mechanisms by which A. alternata avoids killing by reactive oxygen species (ROS) after invasion have begun to be elucidated. The ability to coordinate of signaling pathways is essential for the detoxification of cellular stresses induced by ROS and for pathogenicity in A. alternata. A low level of H2O2, produced by the NADPH oxidase (NOX) complex, modulates ROS resistance and triggers conidiation partially via regulating the redox-responsive regulators (YAP1 and SKN7) and the mitogen-activated protein (MAP) kinase (HOG1) mediated pathways, which subsequently regulate the genes required for the biosynthesis of siderophore, an iron-chelating compound. Siderophore-mediated iron acquisition plays a key role in ROS detoxification because of the requirement of iron for the activities of antioxidants (e.g., catalase and SOD). Fungal strains impaired for the ROS-detoxifying system severely reduce the virulence on susceptible citrus cultivars. This paper summarizes the current state of knowledge of signaling pathways associated with cellular responses to multidrugs, oxidative and osmotic stress, and fungicides, as well as the pathogenicity/virulence in the tangerine pathotype of A. alternata.12/2012; 2012:635431. DOI:10.6064/2012/635431